PARIS, France (AFP) — A cryotherapy session in Paris has turned deadly with one woman dying and another hospitalized in critical condition due to a nitrogen leak from a cold chamber, according to French investigators.

The tragedy occurred on Monday evening at a small sports center located in the 11th district of the French capital, according to a police source.

According to initial findings, a nitrogen leak from the cryochamber is thought to have caused the poisoning, a source close to the investigation said. An employee of the gym, who was in her late 20s, died.

“An investigation into the cause of death has been launched,” the Paris public prosecutor’s office said.

Three people who were in contact with the victims and provided first aid also received treatment, the police source said.

The gym, where 150 people were present when the tragedy struck, was evacuated shortly afterwards.

Agence France-Presse journalists present at the scene on Monday evening saw a body covered with a white sheet being carried out of the sports center.

Screens were set up to conceal what was happening in front of the door to the gym and along the pavement.

Nitrogen is a colorless, odorless gas that is commonly used in cryotherapy.

During a session in a walk-in chamber a person is exposed to temperatures below 100° Celsius for a recommended time of no more than three minutes.

Advocates say whole body cryotherapy is effective in reducing muscle soreness, stress, rheumatism and various skin conditions.

Star athletes and celebrities have used whole body cryotherapy as an alternative to ice packs and cold water baths.

But many experts warn that the treatment has not been proven to be medically sound and are urging further research to determine the short- and long-term effects.